1. Field of the Invention
The present invention relates, generally, to a method for data transmission between two communications devices via a packet-oriented communications network and, more specifically, to a transmission system for transmitting time-slot-oriented data between an exchange termination device ET and a line termination LT. According to the terminology of the ITU-T G.960 (3/93) standard, “access digital section for ISDN basic rate access” (international Telecommunication Union), in particular pages 2 and 3, the invention correspondingly relates to data transmission on the “V reference point”.
2. Description of the Prior Art
A method is known, for example, from US patent specification U.S. Pat. No. 5,793,766 for two-way data transmission between communications devices supporting a time-slot-oriented data transmission protocol, having channel-specific information segments, via a packet-oriented communications network. Here, the data packets used to transmit data via the packet-oriented communications network are subdivided into a first sub-packet and into a second sub-packet, data of different channel-specific information segments in each case being transmitted in a first data packet.
A transmission system for transmitting time-slot-oriented data between an exchange termination device and a line termination normally forms part of a communications system which has a switching device and subscriber connection devices. The subscriber connection devices have subscriber interfaces for connecting communications terminal devices to the communications system. According to the ITU-T G.960 standard, the subscriber connection devices are connected via a line termination and an exchange termination device to the switching device of the communications system. A communications system of this type serves to set up or clear down narrowband communications connections between communications terminal devices connected to the subscriber connection devices and to enable narrowband communication (for example, voice or data communication) between the communications terminal devices.
In modern communications systems, data transmission normally takes place between the exchange termination device and the line termination on the basis of the time-slot-oriented data format IOM-2 (ISDN Oriented Modular Interface), which is formed from a periodic sequence of channel-specific information segments; referred to below as the time division multiplex channel. One time division multiplex channel is normally allocated, in each case, to each subscriber interface of a subscriber connection device.
In modern communications technology, there is a need for broadband transmission of information; for example, still and moving pictures in videotelephony applications or large data volumes on the Internet. This increases the importance of transmission technology for high and variable data transmission rates (greater than 100 Mbit/s), which take account both of data transmission requirements (high speed with variable transmission bit rate) and voice data transmission requirements (maintenance of temporal correlations in the case of data transmission via a network), in order to be able to integrate the separate networks currently existing for the different purposes into one network. A known data transmission method for high data speeds is referred to as Asynchronous Transfer Mode (ATM). Data transmission on the basis of the Asynchronous Transfer Mode currently enables a variable transmission bit rate of up to 622 Mbit/s.
In the cell-based data transmission method known as Asynchronous Transfer Mode (ATM), fixed-length data packets, referred to as ATM cells, are used for data transport. An ATM cell includes a five-byte cell header containing switching data relevant to the transport of an ATM cell, and a 48-byte user data field, referred to as the ‘payload’.
Data transmission via an ATM-based communications network generally takes place in “virtual paths” or in virtual channels contained in the virtual paths. To do this, connection tables with switching information including a “virtual channel identifier” and a “virtual path identifier” are created when a connection is set up, before the start of the actual user data transmission, by exchanging signaling information in the respective ATM network nodes of the ATM-based communications network. In the connection tables, a “VCI value” is allocated to the virtual channel identifier and a “VPI value” is allocated to the virtual path identifier. The switching information recorded in the connection table of an ATM network node determines how the virtual paths or virtual channels contained in the virtual paths of the incoming and outgoing connections on the ATM network node are allocated to one another by the signaling; i.e., which input is linked by the switching system to which output of the ATM network node. The cell header of ATM cells transmitted via these virtual connections (virtual paths and virtual channels) essentially contains switching data including a VPI value and a VCI value. The ATM cell header data is processed at the input of an ATM network node; i.e., the switching data disposed therein is collected and evaluated. The ATM cells are then switched through the ATM network node using the switching information stored in the connection table to an output of the ATM network node representing a specific destination.
In the German patent application with the official reference 198 45 038.9, a transmission system between an exchange termination device and a line termination has already been proposed in which the transmission is implemented via an ATM-based communications network. Here, subscriber interfaces are made available in order to connect communications terminal devices via ATM transfer units, which are frequently referred to in the literature as the “ATM hub”, which are connected to the ATM-based communications network. The exchange termination device of the communications system and the line termination implemented by the ATM transfer unit in each case have an ATM connection unit, via which, on the one hand, a connection to the ATM-based communications network is implemented and, on the other hand, a two-way conversion is carried out between the IOM-2 data format, normally provided for data transmission between the exchange termination device and the line termination, and the ATM data format.
The two-way conversion between the time-slot-oriented IOM-2 data format and the cell-based ATM data format takes place according to two different conversion methods. According to the first conversion method, on the basis of the specification CES 2.0 of the ATM Forum, the time-slot-oriented data is packeted byte-by-byte in ATM cells according to the first ATM adaptation layer AAL1. The ATM adaptation layer AAL (ATM Adaptation Layer) serves to adapt the ATM data format (corresponding to Layer 2 of the OSI reference model) to the network layer (Layer 3) of the OSI reference model (Open System Interconnection). According to the second conversion method, the time-slot-oriented data is packeted byte-by-byte into ATM cells substructured according to the second ATM adaptation layer AAL2.
An object of the present invention, therefore, is to offer an alternative method which two-way data transmission can take place between the communications terminal devices and the switching system.